A furnace may have a process chamber in which a load is heated by products of combustion. The products of combustion are generated by reactants that are discharged from burners that fire into the process chamber. Each burner receives unignited streams of fuel and oxidant, and may have a reaction zone in which the fuel and oxidant are ignited and combust to form a flame. The reaction zone has an outlet through which the flame projects from the burner into the process chamber, and may be tapered at the outlet to stabilize the flame.
When the process chamber reaches an elevated temperature at or above an auto-ignition temperature of the fuel, it may be desirable to combust the reactants in a diffuse mode without a stabilized flame projecting from burner into the process chamber. A diffuse mode can be initiated by extinguishing the flame and continuing to discharge the reactants from the burner into the process chamber. As long as the process chamber has a temperature at or above an auto-ignition temperature of the fuel, the reactants will then ignite and combust in a diffuse mode in the process chamber.